1. Field of the Invention
The present invention relates to a voltage buffer, and more particularly to a voltage buffer for suppressing non-ideal effect by changing the threshold voltage of a transistor.
2. Description of Related Art
A voltage buffer is used to transmit voltage signal, enhances the driven capability and avoids loading from influencing the output of circuit. FIG. 1 is a circuit diagram illustrating a conventional power amplifier. Please referring to FIG. 1, a voltage buffer 100 includes a first, second and third transistors M1, M2 and M3, a current source IREF, an input voltage VIN and an output voltage VOUT. The first transistor M1 is coupled between a system operating voltage VCC and the second transistor M2. The current source IREF is coupled between the system operating voltage VCC and the third transistor M3. And the control terminals of the second and third transistors 120 and 130 are coupled to each other.
The voltage buffer 100 takes the first transistor 110 as a source follower and takes the second and third transistors 120 and 130 as a current mirror. From the transistor current formula, a bias current IB=I110=K(VIN−VOUT−VTH,110)2 could be deduced, where VTH,110 is the threshold voltage of the first transistor 110 and K is a constant. That is, when the bias current IB remains invariant, the output voltage is proportional to the input voltage VIN.
However, with the evolution of the process, the bias current IB is prone to vary by the drain-to-source voltage. In order to mitigate the non-ideal effect generated by the variation of the bias current, a plurality of circuit design which improve the linearity by controlling the bias current are provided. However, the design of controlling the bias current directly will make the high-frequency circuit unstable, and other compensation circuits are added to stabilize the circuit. Consequently, both the complexity and cost of the circuit design are increased.